It is known in the prior art that the Federal Communications Commission (FCC) has allocated frequency channels, each having a 6-MHz width, for commercial television. Channels 2 through 13 are known as the very high frequency (VHF) channels and span a frequency range of 54-216 MHz. Channels 14-83 are known as the ultrahigh-frequency (UHF) band and span the frequency range of 472-890 MHz.
As is known in the prior art, a TV receiving antenna should have sufficient gain and present a good impedance match in order to deliver a TV signal to a transmission line and subsequently to a TV with a clear picture and sound. The TV receiving antenna should provide these properties over all TV channels of interest, more particularly the complete 54-890 MHz frequency range. Such TV receiving antennas typically fall into two categories, indoor antennas and outdoor antennas. The present discussion is limited to indoor antennas.
The most common configuration of an indoor antenna consist of two antennas, one for receiving all VHF channels and one for receiving all UHF channels. The most popular indoor VHF antennas are extendable monopole and dipole telescoping cods (rabbit-ear antennas). A disadvantage to these antennas is that they must be adjusted in length and oriented for best signal strength and to minimize "ghost" images for each channel to be received. Another disadvantage to these antennas is that they are large, each rod particularly on the order of a quarterwave length of the operating frequency band. The two rods of the dipole antenna thereby make up a half wavelength dipole antenna. Thus, a disadvantage of the prior art VHF antennas is that they are extremely burdensome to operate and take up substantial space.
There are also known in the prior art, several configurations of indoor UHF antennas, including a circular loop and a triangular dipole. A problem with the circular loop and the triangular-dipole antennas are that they have low gain. In addition, they also have the problem that they have to be adjusted to minimize "ghost" images for each TV channel to be received.
The single turn circular-loop antenna is a popular UHF antenna primarily because of its low cost. The single-turn loop is a resonant structure, in which the entire UHF frequency band of operation is possible by using a 20.3 centimeter diameter single loop construction such that the circumference of the loop varies across the frequency operation band from a wavelength at 470 MHz to 1.7 wavelengths at 806 mHz, The single-loop antenna has a bidirectional antenna pattern with a maximum directivity along the loop axis. A single loop oriented in a vertical manner and fed at the bottom is thus horizontally polarized. A problem with the single loop antenna is that the input resistance and reactance, respectively, of the single-turn circular loop varies across the frequency band of interest. Consequently, a measured voltage standing wave ratio (VSWR) while close to one near the center of the band, increases to approximately 4.0 at both ends of the frequency band. Thus, there is a significant degradation in performance at the ends of the frequency band of operation.
The aforementioned rabbit-ear antennas are typically available with either a 75- or a 300-ohm impedance. The single-loop UHF antenna is most commonly designed with a balanced 300-ohm impedance. Thus, a popular VHF-UHF combination antenna consists of a continuation of the VHF rabbit-ear dipole antenna and the UHF single-loop antenna mounted on a fixture containing a switchable impedance-matching network. As discussed above, the problem with such an antenna is its size and its cumbersomeness of use.
Accordingly, the present invention is directed to solving the cumbersome operation and size problems associated with the prior art antennas. In addition, the present invention is directed to solving the performance problems associated with the prior art antennas.